Tricks boy's acquired — they're inflammatory! (9)
I believe the answer is:
arsonists
'tricks' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'boy's acquired they're inflammatory' is the wordplay.
I cannot quite see how this works, but
'boy' could be 'son' ('son' can be a synonym of 'boy') and 'son' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for arsonists that I've seen before include "people with a burning desire" , "Criminals" .)